Food is considered spoiled when an undesirable change in the color, flavor, odor or texture has occurred. The onset of food spoilage is rather indefinite. It is a gradual process occurring because of poor sanitation, enzymatic or chemical reactions, improper temperature controls, microbial growth or physical abuse.

Microbial spoilage is the major cause of food spoilage. A food product that starts with 100 microorganisms per gram may have a shelf life of 12 days before it develops off odors, slime and spoilage. When the initial number is 5,000 per gram, the shelf life of that same foodstuff may be shortened to seven days. Since so much depends on the initial number of bacteria, temperatures and handling practices, a specific shelf life for a category of food products is difficult to determine.

Enzymes help speed up or slow down chemical reactions, act as transports for foods, and are a normal constituent of foods. Enzymes can be inactivated by heat, which is the reason for blanching vegetables; or they can be inactivated by cold temperatures below 40o F, which is the reason for placing vegetables under refrigeration.

Vacuum packaging, over-rapped or tray packaging, freezing, drying, canning of food products, use of antioxidants etc. are the various techniques to preserve the foods to retard the spoilage.

INTRODUCTION

Foods are described as spoiled if organoleptic changes make them unacceptable to the consumer. These organoleptic characteristics may include changes in appearance (discoloration), the development of off-odours, slime formation, changes in taste or any other characteristic which makes the food undesirable for consumption. Whilst endogenous enzymatic activity within muscle tissue post-mortem can contribute to changes during storage, it is generally accepted that detectable organoleptic spoilage is a result of decomposition and the formation of metabolites caused by the growth of microorganisms [2].

The signs that food is spoiling are:

Odour: "off odours" are smells (sometimes like rotten eggs) that are produced when bacteria break down the protein in food, (usually fatty foods). This process is called putrefaction. Taints due to flavour change may also occur.

Sliminess:Food becomes slimy as the bacterial population grows.

Moulds may also form slimy whiskers.

Discolouration: Foods can become discoloured by microbial growth. Some moulds have coloured spores that give the food a distinctive colour, for example, black pin mould on bread, or blue and green mould on citrus fruit and cheese.

Souring: Foods go sour when certain bacteria produce acids. A common example is when milk sours from the production of lactic acid.

Gas: Bacteria and yeasts often produce gaseous by-products that can affect food. It may be noticed that meat becoming spongy, or packages and cans swelling or having a popping or fizzing sound on opening.

According to the cause of the spoilage, types of food spoilage fall into two major categories. Microbial spoilage is caused by microorganisms and their products; non-microbial spoilage can be caused by foreign material in the foodstuff or by enzymes that occur in the foodstuff naturally [6].

Spoilage of any particular food will be by those organisms most suited to the conditions in and around that food. The three main groups of concern are

Bacteria: Bacteria are the main and an important cause of food spoilage. They thrive where food and water are present and the temperature is suitable, as in the nose, throat, skin, bowel and lower urinary tract of man and animals. They are single cell organisms usually having a definite outer envelope or capsule for protection. They multiply by dividing into two, which can occur very quickly, (eg. every 20 minutes). They can actively move and some link themselves together in chains or in bunches.

To resist harm, some bacteria can form spores (tough reproductive cells that are able to survive under adverse conditions), that can resist damage by heat (as in cooking), by cold (as in freezing) and by chemicals such as disinfectants. A spore can survive in dust, on vegetation and in soil for weeks, months or even years until it finds itself in a suitable environment for growth.

Fig 1: Growth curve

Bacteria need about four hours to adapt to a new environment before they begin rapid growth. In handling food, this means there is less than four hours to make a decision to cool the food, heat it, or eat it. For example, when chickens arrive at the dock of a fast food outlet, or at a restaurant or at home, it must be decide whether to heat and eat them, to refrigerate them at a low temperature (chickens freeze at 28 degrees F) for a short period of time, or whether to wrap and freeze the chicken for a longer period of time. If it is not decided, the bacteria will enter the log phase of growth where bacteria grow rapidly and cause food to spoil. Bacteria produce the slime, toxins, off colors and odors associated with food spoilage in the log phase of growth. It should be remembered that the four hours bacteria remain in the log phase is approximate and cumulative.

As microorganisms grow, they tend to form colonies. These colonies are made up of millions of individual cells. Once a colony forms, the food available to each cell is limited and excretions from these millions of cells become toxic to a microbe. This is the stationary phase. Some of the cells now begin to die.

Viruses:Viruses are organisms much smaller than bacteria. In their pre-infective stage they are just like a chemical with none of the requirements for life, but once in a living cell they take over and begin to multiply. They can grow only in living tissue, but can be carried in food from one person to another.

Fungi: Yeasts are single cell organisms much larger than bacteria and can be found in the soil, on plants and on the skin and body of man. They multiply by forming offspring as buds which grow and then detach themselves. Some can produce disease, some cause skin infections in man and others cause diseases in plants. Some yeasts spoil food, but beneficial uses are in the making of beer, wine and bread.

Moulds grow as single cell filaments that can branch together making a strongly knit structure like a mat, that can often be seen with the naked eye. Usually they look fluffy, being a familiar sight on foods like jam, cheese and bread. They multiply by producing clusters of dry spores which are blown by the air like seeds. Many moulds spoil food and a few can cause disease in plants and man, but beneficial uses are in the ripening of cheeses and production of antibiotics [3].

Control of microbial spoilage

Microbial spoilage is the major cause of food spoilage. It occurs as a result of contamination of food by microorganisms, provision of a suitable environment for their growth, and degradation of the foodstuffs. The micro-organism can grow

At temperatures between -7 to around 70°C

Over a pH range from 0 to 11

In the presence or absence of oxygen

At water activities above about 0.6

To retard microbial buildup, the following parameters must be controlled:

Source:Foodstuffs are naturally contaminated with microorganisms. To keep numbers of microorganisms as low as possible, fresh foods should be washed such as fruits and vegetables to physically remove as many microorganisms as possible. Processors of foods must keep their plants and equipment as clean as possible to provide clean work surfaces that come in contact with food. Every effort should be made to keep the initial numbers of microorganisms as low as possible.

Fig 2: Variation in Bacterial number with time

Figure 2 shows why it is needed to keep initial numbers low. A food product that starts with 100 microorganisms per gram may have a shelf life of 12 days before it develops off odors, slime and spoilage. When the initial number is 5,000 per gram, the shelf life of that same foodstuff may be shortened to seven days. Since so much depends on the initial number of bacteria, temperatures and handling practices, a specific shelf life for a category of food products is difficult to determine [8].

Initial numbers can be kept low by practicing good personal hygiene, by sanitizing equipment and controlling temperature, and by using chlorinated water where possible.

Food:Like every other living thing, bacteria require food to live. They need only very small quantities. Some protein or fat left on the wall of a processing plant, grease on the blade of a knife or saw, or food residues on the wheel of a can opener or on a cutting board are a feast for microorganisms as well as for larger pests. Any equipment that may contact food (food contact surfaces) must be cleaned thoroughly.

Moisture:Every living thing requires moisture, and bacteria are no exception. Food that requires refrigeration is usually very high in moisture content. Moist food left over for long periods of time provides adequate moisture for growth [1].

Figure 3 shows a typical bacterial cell. Its surface is rough, similar to a sponge's surface; its only means of obtaining food is by absorption similar to that of a sponge. Enzymes manufactured inside the cell move out onto foodstuffs, combine with the food, and return to the cell. This process cannot be accomplished without moisture. This is why foods such as dried milk, dried soups and cereals do not spoil microbiologically. The bacteria are present, but they can't eat.

Different bacteria require different temperatures for maximum growth. Some bacteria, called psychrophiles, will grow at refrigerated temperatures. Others, called mesophiles, will only grow at moderate temperatures. Warm-loving bacteria, called thermophiles, grow at temperatures above 140 degrees F. At temperatures above and below the optimum, they grow and reproduce at a slower rate. Food spoilage bacteria grow best at environmental temperatures of 70 to 100 degrees F. A generation time is the amount of time it takes for a bacterium to reproduce itself. The shorter the generation time, the faster food spoilage will occur [5].

Fig.3: Typical bacterial cell

Food may spoil as a result of chemical changes within the food itself or by a reaction between the food and the packaging material. Rancidity is caused by a chemical reaction that breaks down the fatty acids in fat to smaller molecular weight fatty acids and, at the same time, releases certain odiferous products. Washers, bolts, nuts and various other items have been found in canned foods. This generally occurs when a maintenance person makes a repair on the line and uses a can for holding parts. The can stays on the line and is filled with the product. Although the product is retorted and sterile, it is aesthetically undesirable to find metallic parts in canned foods [4].

Enzymes are chemicals produced by all living things. They help speed up or slow down chemical reactions, act as transports for foods, and are a normal constituent of foods. For instance, as a banana matures, the color changes from green to yellow to brown to black. The change is caused by the enzymes (chemicals) in the banana. The ripening, then softening, of other fruits such as apples, peaches and tomatoes is another example of enzymatic action. Enzymes can be inactivated by heat, which is the reason for blanching vegetables; or they can be inactivated by cold temperatures below 40 degrees F, which is the reason for placing vegetables under refrigeration [9].

Bacteria also produce enzymes that break down food and allow them to obtain nutrients through their cell walls. Therefore, lowering the temperature reduces the rate of enzyme action as well as the rate at which bacteria can multiply. Refrigeration increases the time required to spoil food.

As the number of bacteria increases, the amount of enzymes produced increases. Higher temperatures can cause increased enzymatic activity. With large numbers of bacteria and high temperatures, a food will spoil very rapidly. When bacterial contamination is high and the storage temperature is low, a food will keep for a moderate period of time; when the bacterial contamination is low and the storage temperature high, food will keep for a moderate period of time. However, if the contamination of bacteria is low and the storage temperature kept low, the food product will have the longest possible shelf life [7].